(1) Field of the Invention
The present invention relates to a composite developer comprising a mixture of magnetic toner particles and non-magnetic toner particles. More particularly, the present invention relates to a composite developer comprising a mixture of magnetic toner particles and non-magnetic toner particles, in which both the particles have a friction chargeability characteristic of the same polarity and both the components therefore are consumed at the same rate at the time of development, with the result that no change is caused in the composition of the developer.
(2) Description of the Prior Art
In the electrophotographic reproduction or electrostatic recording, a magnetic brush development method is widely used as means for developing an electrostatic latent image. A two-component type developer and a one-component type developer are known as the developer that can be used in this magnetic brush development method. In each case, the developer is held on a non-magnetic sleeve provided with a magnet in the interior thereof to form a magnetic brush and a visible image is formed by sliding contact of this magnetic brush with an electrostatic latent image supporting-substrate.
The developer to be used in this developing method should have both the magnetic attractability and electrostatic attractability. In case of the two-component type developer, a mixture of a carrier comprising a magnetic powder such as iron powder and colored electroscopic fixable particles is used and by friction between the two components, the fixable particles are electrically charged and electrostatic attractability is given thereto. Accordingly, in the known two-component type developer, only the electroscopic fixable particles, that is, toner particles, are consumed, and the composition of the developer is gradually changed. Therefore, it is necessary to conduct troublesome operations of adjusting the composition of the developer at times and exchanging the degraded carrier formed by long-time use with a fresh carrier.
There has recently been proposed a composite developer formed by mixing magnetic toner particles composed of a dispersion of a powdery magnetic material in a binder resin medium with non-magnetic toner particles composed of a dispersion of a coloring material in an electrically insulating binder resin medium. Also in the developer of this type, non-magnetic toner particles are electrically charged by frictional contact with magnetic toner particles, and the non-magnetic toner particles are consumed in larger quantities for the development. Accordingly, also the composite developer of this type is inevitably defective in that the composition of the developer is readily changed while it is used.